Nitrous oxide increased the turnover rate of acetylcholine in diencephalon and septum and reduced the acetylcholine content in striatum. These effects were reversed by naltrexone suggesting that nitrous oxide releases or potentiates an endogenous analogesic or opioid. BIBLIOGRAPHIC REFERENCE: Ngai, S.H., Cheney, D.L. and Finck, A.D.: Effects of halothane, enflurane and ketamine on acetylcholine concentrations and turnover in rat brain structures. J. Anesthesiol., in press, 1977.